A relay contact symbol represents the physical switching mechanism of a relay in an electrical schematic, completely distinct from the electromagnetic coil that actuates it. While the coil is drawn as a rectangle or circle, the relay contact symbol illustrates the conductive path—whether Normally Open (NO), Normally Closed (NC), or Changeover (CO/DT)—and dictates how current flows when the coil is energized. Because North American and international drafting standards differ visually, misreading these symbols can lead to catastrophic wiring faults, such as shorting a motor reversal circuit.
The Master Relay Contact Symbol Reference Table
The table below maps the most critical relay contact symbols to their IEC 60617 and NEMA/ANSI Y32.2 designations. Use this as your bench-side cheat sheet when tracing control circuits.
| Contact Type | IEC 60617 Symbol Description | NEMA / ANSI Symbol Description | Standard Designation | Practical Application |
|---|---|---|---|---|
| Normally Open (NO) | Line with a gap; diagonal slash pointing away from the gap | Line with a gap; parallel slash angled upward | Form A / SPST-NO | Start buttons, solenoid valves, indicator lights |
| Normally Closed (NC) | Line with a gap; diagonal slash crossing completely over the line | Line with a gap; parallel slash crossing downward | Form B / SPST-NC | Stop buttons, safety interlocks, E-stop circuits |
| Changeover (CO) | Common line branching to both an NO and NC contact | Common line branching to both NO and NC with specific slash angles | Form C / SPDT | Motor reversing, toggle logic, alarm state switching |
| Make-Before-Break | Form C symbol with a bridging line indicating overlap | Form C symbol with an overlapping center node marker | Form D / SPDT (MBB) | Audio routing, uninterruptible power transfer |
| Time-Delay ON | NO or NC symbol with an 'X' or arrow on the slash | NO or NC symbol with an inward-pointing arrowhead on the slash | Form A/B (Timed) | Soft-start delays, cooling fan run-on timers |
Regional Standards and the 'Rows People Get Wrong'
Electrical schematics are not universal. The standard that applies to you depends on your region and the origin of the machinery you are servicing.
- IEC 60617 (International/EU/Modern Global): Uses clean, geometric lines. The slash for an NO contact points diagonally away from the hinge. This is the standard for 90% of modern PLCs and European-imported machinery.
- NEMA ICS 1 / ANSI Y32.2 (North America): Uses a more stylized, angled slash. Often found in legacy US manufacturing plants, Allen-Bradley control panels, and older HVAC systems.
- BS 3939 (Old UK): Largely superseded by IEC, but you will still encounter it in British facilities built before the late 1990s. It features distinct, heavier line weights and slightly different hinge notations.
The Rows People Get Wrong
When tracing complex ladder logic, these specific symbol variations cause the most bench and jobsite errors:
- The 'X' on the Slash (Time-Delay vs. Latching): In IEC, an 'X' drawn through the diagonal slash of a relay contact symbol indicates a time-delay contact (e.g., ON-delay or OFF-delay). Many technicians mistakenly interpret this 'X' as a mechanical latching relay. If you wire a standard DPDT relay into a circuit expecting a timed OFF-delay, the machine logic will fail immediately upon power loss.
- Form C Drawn as Separate Form A and Form B: Drafters sometimes draw a Form C (SPDT) relay as two separate symbols (one NO, one NC) linked by a dotted mechanical line. If you don't notice the dotted line, you might assume they are two separate relays and wire the commons to different voltage potentials, destroying the internal wiper when the coil pulls in.
- Break-Before-Make vs. Make-Before-Break: A standard Form C relay is break-before-make. If the schematic requires make-before-break (to prevent an open circuit during transfer, common in CT secondary circuits), the symbol will feature a bridging line. Swapping in a standard Omron MY2N here will cause a dangerous open-circuit spike on a current transformer.
Safe Interpretation When Markings Are Faded or Missing
On the jobsite, relay covers get lost, and the stamped pinout diagrams on the side of an Allen-Bradley 700-HA or an Omron G2R-2-SND fade after years of heat cycling. Never guess the pinout based solely on the schematic symbol order; physical pin layouts rarely match the left-to-right drawing order of a schematic.
If the physical relay markings are illegible, use a digital multimeter (like a Fluke 87V) in continuity/ohms mode to map the pins safely while the relay is de-energized and removed from the socket.
Step-by-Step Pin Mapping Procedure
- Locate the Coil: Measure resistance across pairs of pins. The coil will show a specific DC resistance (e.g., ~650 Ω for a 24VDC Omron MY2N, or ~150 Ω for a 12VDC Finder 55.34). On an 8-pin octal base, the coil is almost always pins 2 and 7. On a 14-pin, it is pins 13 and 14.
- Identify the Commons (C): Set your meter to continuity. Find the pins that show continuity to two other separate pins. Those are your Common terminals. (On an 8-pin, they are pins 1 and 8).
- Map NC and NO: With the relay at rest (de-energized), the pin showing continuity to the Common is the Normally Closed (NC) contact. The pin showing an open circuit (OL) to the Common is the Normally Open (NO) contact.
- Verify Mechanically: Use a small jumper wire to apply the rated coil voltage (e.g., 24VDC) directly to the coil pins. You should hear the armature click. Re-test the contacts; the NO should now show continuity, and the NC should show OL.
Frequently Asked Questions
What is the difference between a relay contact symbol and a coil symbol?
The coil symbol represents the electromagnetic actuator and is universally drawn as a rectangle (IEC) or a circle with a diagonal line (NEMA), labeled with a designator like 'K1' or 'CR2'. The relay contact symbol represents the physical metal switches moved by that coil. They are drawn separately on the schematic, often on entirely different pages of a large panel drawing, linked only by the shared alphanumeric designator (e.g., K1-NO, K1-NC). The coil consumes power; the contacts switch the load.
How do I identify a time-delay relay contact symbol on a schematic?
Look for an arrowhead or an 'X' intersecting the diagonal slash of the contact symbol. In IEC 60617, an arrow pointing inward toward the hinge indicates an ON-delay (the contact waits to close after the coil is energized). An arrow pointing outward away from the hinge indicates an OFF-delay (the contact stays closed for a set time after the coil loses power). In NEMA, the arrowhead direction follows the same logic but uses a distinct triangular shape rather than a simple line.
Why do some schematics use 'SPDT' while others use 'Form C' for the same relay contact?
They describe the exact same physical configuration: a Single Pole, Double Throw switch with a Common, NO, and NC terminal. 'SPDT' is the general electronics and switchcraft terminology (used for toggle switches and slide switches), while 'Form C' is the specific NEMA/industrial control nomenclature for relay contacts. 'Form A' equals SPST-NO, and 'Form B' equals SPST-NC. If you are ordering replacement relays from industrial suppliers like AutomationDirect or Digi-Key, searching by 'Form C' or 'DPDT' (which is two Form C contacts on one coil) will yield the correct industrial control relays.
For deeper study on schematic drafting standards, refer to the Electrical Engineering Portal's guide on relay symbols and the All About Circuits breakdown of relay logic.
